ocamlbuild: polish the interaction between Param_tags.init() and plugin execution

Before this path, [Param_tags.init()] was run before
[Plugin.execute_plugin_if_needed()]. It had the downside of making
plugin compilation warn about any unknown parametrized tag present in
a _tags file, even those that would be defined by the plugin
itself. The fix is to [really_acknowledge] only the tags used for the
plugin compilation, and delay the full [Param_tags.init()] call to
after plugin execution.

This was discussed in detail in PR#5680, comment 0010186.

git-svn-id: http://caml.inria.fr/svn/ocaml/version/4.01@14013 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
ocamlbuild testsuite: test modular plugin compilation

git-svn-id: http://caml.inria.fr/svn/ocaml/version/4.01@14014 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02